With the passage of time, asphalt pavement deteriorates in response to the influence exerted by a variety of factors. These include weather, reaction to traffic load on the pavement surface, change in the composition and physical properties of the asphaltic component of the pavement, faulty base courses and improper drainage.
The impact of these various factors on the asphalt pavement manifests itself variously in cracks, pot holes, dips, bumps, ruts and other irregularities and discontinuities. Ultimately, economic considerations and safety factors dictate that some form of maintenance be performed on the degraded road.
In the past, before the advent of energy shortages and before the spiralling costs of materials and labor became controlling factors in maintenance decisions, a wide variety of repair and reconstruction procedures were available to the authority responsible for road maintenance. These procedures ran the gamut from radical road rebuilding to moderately expensive resurfacing with a 11/2 to 2 inch overlay of asphaltic concrete, to minimal maintenance involving patching and/or applying a seal coat.
Many of the options previously open to highway engineers are now closed due to prohibitive costs and/or lack of necessary materials.
In recognition of these problems, attempts have been made to develop asphalt pavement reconstruction methods which maximize the reprocessing and reworking of aged, weathered asphalt pavement, and in this manner, minimize labor, transportation and new material requirements. Exemplary of these procedures are the teachings of U.S. Pat. No. 3,361,042 issued Jan. 2, 1968, which describes a method of reconditioning a highway of asphalt concrete by heating the concrete, scarifying it, mixing the materials, planing, leveling, and kneading it, and finally, rolling and compacting it into a reconditioned surface. According to the patent, the steps can be accomplished with or without the addition of new asphaltic paving materials.
Procedures of the type described in U.S. Pat. No. 3,361,042, although of substantial interest because of economic considerations, have met with only limited commercial success. One of the major problems appears to be the inability of such prior art techniques to effect a deep enough scarification to get below cracks, ruts and pot holes, without unduly disturbing the asphalt concrete matrix.
For example, according to the teachings of U.S. Pat. No. 3,361,042, scarification is effected to a depth below which the applied heat softens the asphalt pavement. The unheated material is physically brought to the surface where it is broken up into random rubble and then heated to soften it. This process not only requires the expenditure of substantial mechanical energy to scarify but tends to promote both comminution and segregation of aggregate. Both of these phenomena have the adverse effect of decreasing the strength, stability and load bearing capacity of the asphalt pavement.
Thus, the Asphalt Plant Manual, 3rd Edition, published by The Asphalt Institute, states at p. 54 that undue segregation of an asphalt mix results in nonuniform distribution of the material in the pavement, and this can lead to a patchy appearance as well as early structural distress. Similarly, Highway Engineering, 3rd Edition, by Ritter and Poquette, published by The Roland Press Company, 1967, points out at p. 561 that even excessive rolling can result in the crushing of aggregate particles or the breaking of the asphalt bond, which is, of course, undesirable.